Nickel hydroxide (Ni(OH)2) represents the electrochemically active material of the positive electrode in nickel cadmium and nickel metal hydride batteries.
Since bivalent nickel hydroxide does not conduct electrons, conductive agents in powder form are generally added. Thus, when the bulk material mixture is compressed, a three-dimensional conductive structure is formed which binds the nickel hydroxide particles with regard to electrons. In bulk material electrodes such as those used in button cells, fine nickel powder is added to the nickel hydroxide to obtain a conductive bulk material.
In the past, graphite was used as the conductive agent in Ni/Cd cells. However, it was susceptible to destruction by oxidation. In electrodes with a metal foam structure, the nickel foam is used as the conductive structure, with its pores being filled with an aqueous suspension which contains nickel hydroxide. Cobalt compounds in the form of CoO, Co(OH)2 or metallic cobalt are added to the positive bulk material to form a fine conductive structure which covers the particles like a network. These compounds are soluble in the electrolyte and, when the cell is first charged, are changed to CoOOH, which conducts electrons and binds the Ni(OH)2 particles in the desired manner.
To make it possible to charge the positive electrode, the potential layers of the oxidation of the Ni(OH)2 to form NiOOH (charging reaction) and the development of oxygen are important:Ni(OH)2+OH−→NiOOH+H2O+e−  (1)4OH−O2+2H2O+4e−  (2)
A certain amount of overvoltage is required for each of the redox processes (1) and (2) mentioned above, and this varies with temperature, like the position of the respective redox potentials. Inadequate charge absorption occurs in particular at increased charging temperatures and low charging currents since a considerable proportion of the amount of charge supplied is consumed for oxidation of the hydroxide ions. The positive electrode can no longer be fully charged since the charging voltage required for this purpose cannot be achieved due to the amount of oxygen which will be developed at a potential below the required charging voltage. The capacity of the entire cell also falls with the decrease in the charge of the positive electrode.
It is known from EP 0 867 959 A2 that additives of compounds of the elements yttrium, ytterbium, erbium, indium, antimony, barium, calcium and beryllium lead to an improvement in the amount of charge absorbed at increased charging temperatures.
It is also known from EP 0 923 146 A1 that additives of oxides of the elements yttrium, ytterbium, calcium, titanium, niobium and chromium lead to an increase in the overvoltage for oxygen evolution and, thus, to an improvement in the amount of charge absorbed at increased temperatures.
It is further known from EP 0 587 973 B1 that additives of oxides or hydroxides of the elements yttrium, indium, antimony, barium, calcium and beryllium lead to an improvement in the amount of charge absorbed at increased charging temperatures.
It is still further known from EP 0 834 945 A1 that additives of oxides or hydroxides of the elements from the group of lanthanoids lead to an improvement in the amount of charge absorbed at increased charging temperatures.
It would accordingly be advantageous to provide a positive electrode which absorbs a large amount of charge at increased temperatures, and has a high capacity at increased charging temperatures as well.